Actually, with the proliferation of USB-C Power Delivery, this is now possible on modern laptops. RAVPower and Anker both make USB-C PD packs that are right at the aviation capacity maximum that can charge even a relatively hefty laptop for an extra couple of hours. I really enjoy traveling with mine and my HP Spectre X360, although the pack is indeed a bit on the chunky side so I assess whether to bring it based on the specifics of my flights and destination.